1. Field of the Invention
The present invention generally relates to microphones and, more particularly, is directed to a microphone apparatus suitable for reducing an unnecessary noise signal by adaptive signal processing.
2. Description of the Prior Art
In a recorder such as a video tape recorder having a built-in type camera or the like, the microphone picks up and produces unnecessary noise signals generated from an inner (mechanical system) or outer vibration generating source in addition to the desired audio signal.
That is, the microphone picks up vibration (inner vibration) of a driving section of the video tape recorder to produce noise. For example, when a recorder is placed on a desk and when any vibration (external vibration) is applied to the desk, the vibration is picked up as noise.
To solve this problem, the microphone is designed to have directivity with a low sensitivity to the noise and mounted to be positioned as far as possible from the noise source. However, since the noise reduction is not enough, the noise is also reproduced as audible sounds. Also, when an external vibration is directly applied to the microphone, the vibration is picked up as noise.
To overcome this disadvantage, the adaptive signal processing is known, in which the noise signal picked up and produced from the microphone is electrically processed and reduced. As will be understood from FIG. 1, an adaptive filter 11 used in the adaptive signal processing includes (K-1) delay elements (for every clock) 20 . . . and K variable amplifiers 30
The first amplifier 30 (leftmost one in FIG. 1) is directly supplied with a reference signal n; and the succeeding amplifiers 30 are respectively supplied with the reference signal n.sub.1 through the respective delay elements 20.
Assuming that W.sub.k is the coefficient of the adaptive filter 11 and also assuming that t is time and that t-1 is the time of one preceding clock, then the following equation (1) will be established between coefficients W.sub.k,t and W.sub.k,.sub.t-1 : EQU W.sub.k =W.sub.k,t-1 +2.mu..epsilon..sub.t-1 x N.sub.1 k,.sub.t-1( 1)
Then, the coefficients W: are changed, and each time they are changed, a filter associated with the reference signal n.sub.1 is formed.
The adaptive filter 11 is proposed in B. Widrow and S. D. Stearns: "Adaptive Signal Processing", Prentice-Hall, 1985, and in Digital signal processing - advanced course, &lt;adaptive signal processing&gt;, Journal of 35th Technical Lecture Meeting held by Acoustical Society of Japan, etc.
In the adaptive signal processing, however, the amount of noise signal reduction depends on a reference signal necessary for such processing, and thus there is a problem of how to select the reference signal.